Gerry1time
Did a wee in Dirac's toilet
The language here at the time of the Romans is still here, in Wales, Scotland and Cornwall. The Saxons, Angles and Jutes drove the indigenous population into what is now Wales.
It's increasingly looking like they didn't in many places. For example, the Dobunni, the pre-roman celtic tribe round here (Bristol and up into the Cotswolds), seem to have continued on pretty much unchanged, apart from being renamed the Hwicce, right through Saxon times. Theology, boundaries and population all largely unchanged. Even the word Hwicce recognises the celtic belief systems. New words were introduced of course, but many celtic place names still survive in different places, the multiple River Avons being notable examples.
If anything it was the Normans that brought the greatest changes, but then they themselves were originally Viking.